Table of contents

The authors have observed very narrow two-photon millimetre resonances in sodium, corresponding to nS to (n+1)S transitions (principal quantum number n ranging from 32 to 40). The linewidth, of the order of 10 kHz for frequencies around 100 GHz, is transit-time limited. The variation with n of the quantum defects in the nS series is studied. The precision of the experiments can still be improved and applications to metrology and to a direct frequency determination of the Rydberg constant are possible.

The absorption spectra of pure Cs and Rb vapours and a mixture of both have been measured in the wavelength region from 2300 to 3100 AA (photoionisation continuum of the atoms) in the temperature range from 410 to 540 degrees C. Pronounced absorption bands can be detected in this wavelength region. Because the absorption was found to depend proportionally on the product of the number densities of the associated atoms (N_Cs², N_Rb² and N_CsN_Rb) and the energies of many observed bands correspond roughly to the sum energies of two electronically excited species, the bands are interpreted in terms of simultaneous excitation of two atoms due to the absorption of one photon ('continuum-state absorption'). For the Cs-Cs and the Cs-Rb systems analogous bands were also observed within the discrete spectra of the atoms involved ('bound-state absorption'). Enhancement of the cross section for pair absorption near dipole-allowed transitions was found in agreement with theoretical predictions.

A simple method for calculating radial integrals between non-hydrogenic states of moderately low to very high principal quantum numbers is presented. The integrals are calculated in the Coulomb approximation using numerical integration over quantum defect radial wavefunctions provided by the WKB approximation. Sample results for dipole transition integrals agree well (to about 2% at worst) with published values obtained with other techniques except for transitions involving relatively large Delta nu / nu .

Reports a broad pattern of non-Franck-Condon vibrational intensities extending over about 25 eV of the 3 sigma _g photoionisation continuum of N₂. The effect was recently predicted theoretically to arise from shape-resonance-enhanced coupling between electronic and nuclear motion. Here experiment and theory are compared directly for the first time. Qualitative agreement verifies the basic theoretical concept, yet significant quantitative differences indicate the need for further work.

Measurements have been made of the cross sections for collisional quenching of highly excited rubidium nS (12<or=n<or=18) and nD_3/2(9<or=n<or=13) states by ground-state rubidium. The experimental values are shown to be close to the geometrical cross section. The large magnitude of the quenching cross section mainly reflects the long range of the Rydberg electron-alkali interaction.

The excitation of the configurations 2p⁵3s and 2p⁵3p in neon has been calculated in the Vainshtein-Presnyakov-Sobel'man approximation (1964) and for e^-, H⁺, H₂⁺ and He⁺ impact. The results are compared with experiment and the first-order distortion calculation of Briggs and Roberts (1974). Good agreement is found with experiment.

Assuming an unpolarised beam, an unpolarised target and a negligibly small amplitude of the direct ionisation process, the angular distribution of electrons from the decay of autoionising states with an arbitrary ratio between the energy splitting of fine structure levels J, J' and the natural level width Gamma is derived.

For pt.I see ibid., vol.12, no.12, p.1939 (1979). The modulated fluorescence in an optical double resonance experiment, i.e. when an atom interacts with two intense monochromatic laser beams each of which is weakly modulated in amplitude at frequency Omega , is studied in detail. The modulation rate corresponding to the steady-state population of the highest atomic level exhibits resonant structures at the modulation frequencies Omega =0, +or- alpha ₀, +or-2 alpha ₀ due to the dynamical Stark splitting of the various energy levels; alpha ₀ being related to the Rabi frequencies alpha ₁ and alpha ₂ associated with the two lasers and the two atomic transitions, by alpha ₀²= mod alpha ₁ mod ²+ mod alpha ₂ mod ². The peak at Omega =+or- alpha ₀ is the most intense and exhibits splitting when the applied fields are detuned from exact resonance with the respective atomic frequencies. Resonances are observed in the modulation rate of the intermediate-level population at the modulation frequencies Omega =0, +or-2 alpha ₀. In the presence of detunings of the applied fields, additional resonant structures occur at Omega =+or- alpha ₀ which increase in magnitude with higher detunings and split. Explicit expressions for the positions, widths and weights of these resonant components are obtained analytically for intense, resonant fields. Numerical results for general values of the laser field strengths and the detunings are given.

A new formula is proposed for the spin-polarisation degree of photoelectrons, emitted from the ground state of alkali atoms in two-photon ionisation caused by two different light beams. The formula is derived within the fine structure scheme and can be applied to polarised as well as unpolarised atoms, all conceivable experimental geometries, all possible combinations of polarisation states of the exciting and ionising light beams, and for near-resonance with a P doublet intermediate state.

Examples are given which point to a non-trivial role of the geometry in two-photon ionisation with two light beams.

Relative integrated intensities for 11 bands of the gamma system of ZrO have been measured by the technique of photographic photometry. The variation of the electronic transition moment R_e(r) with internuclear separation r is studied and is found to be a constant. The effective vibrational temperature of the source has been derived to be 2550+or-50K.

Classical-trajectory Monte Carlo calculations have been performed for collisions of ions in charge states q=+1, +2, +5 and +10 with hydrogenic atoms in principal quantum levels n=1, 2, 5, 10 and 20. The collision velocity range investigated was 1<or=v/v_e<or=10 where v_e is the orbital velocity of the Rydberg electron (1/n in atomic units). Both charge-exchange and impact ionisation cross sections were calculated with impact ionisation found to be the dominant channel for v/v_e>or=2. For v/v_e>or approximately=5, the sum of the charge-exchange (CEX) and impact ionisation (ION) cross sections may be represented by sigma _CEX+ION(a₀²)=6 pi n²q²/v², where v is in atomic units. Analysis of the electronic levels produced after charge exchange by the ion indicates the capture proceeds into excited levels which tend to preserve the energy and orbital size of the initial Rydberg atom.

5 MeV amu^-1 deuteron and oxygen ions were used to obtain K and L X-ray spectra from Cr, Fe, Cu, Ag, In and Sm targets. From these spectra, K beta K alpha intensity ratios and K alpha and K beta X-ray energy shifts were determined for all targets, as well as the L beta /L alpha and L gamma /L alpha intensity ratios for Sm. An examination of the projectile Z(Z₁) dependence on the K beta /K alpha ratios of Ag, Cu and, for the most part, of Fe showed a linear relation for all known results. Both the K alpha and K beta energy shift for Fe were found to vary as Z₁³2/.

Excitation cross sections for the interaction of Na, K and Rb with Hg have been measured by the observation of the light emitted from a scattering chamber. The spectra of this light are given for various collision energies. In these measurements the excitation of the resonance doublet of the alkali atoms is found to be dominant. For these states the energy dependence and the absolute value of the cross sections are reported. By a polarisation analysis the cross sections for the magnetic substate of the P_3/2 component are determined.

The classical trajectory method of Abrines and Percival (1966) has been extended to relativistic velocities of the charged particles. The method is based on the solution of the classical relativistic Hamilton equations of motion for a three-body system consisting of a target nucleus, a charged projectile and a single electron. The electromagnetic fields are taken to be of Lienard-Wiechert type with extended charges of the nuclei. The theory has been applied to calculate the cross section, mean impact parameter and angular distribution for the ionisation of a singly occupied K shell in U-U collisions at E_lab=1 and 10 GeV/n.

Charge-transfer cross sections from the ground state of atomic hydrogen into the final 1s, 2s and 2p states of the He⁺ ion, by alpha particles are calculated in the Coulomb-Born approximation. The results are compared with the existing experimental findings and other theoretical calculations. Throughout the energy region of the incident projectile, the present calculated results are found to show a trend similar to other results obtained by applying many-state close-coupling calculations. The present two-state calculated results are in satisfactory agreement with the existing many-state calculated results as well as with the available experimental findings.

The phase-integral interpretation of the two-state exponential model within the impact parameter formulation is applied to the H-Zn²⁺, H-Cd²⁺ and H-B²⁺ charge-transfer processes confirming the importance of the pseudo-crossing in the results of recent experiments. The importance of the bending of the double Stokes' line in the close curve-crossing region is also confirmed. This approach is further developed and applied to H-Mg²⁺ charge transfer, in which a distinct non-crossing transition region, rather than the pseudo-crossing, is found to be of importance; reasonable agreement with earlier close-coupling results is obtained. It is also applied to the 5g sigma -4f sigma pseudo-crossing of the H-C⁶⁺ charge-transfer process, assuming only knowledge of the exact adiabatic energies and the maximum of the radial non-adiabatic matrix element at real internuclear separation. Good agreement with previous two-state calculations is obtained.

Examines dielectronic recombination through the forbidden levels which are not usually considered. The rate of this process is of the same order of magnitude as that of photo-recombination. The process is of importance for a low density plasma with a rather low electron temperature and high degree of ionisation (for example: H II regions and planetary nebulae). Detailed calculations are made for O II.

In a cross-beam experiment electrons are scattered elastically from xenon atoms. The dependence of the relative cross section on collision energy over the range 2-300 eV and on scattering angle over the range 30 degrees -120 degrees is measured. Very satisfactory overall agreement is found with the experimental work of Mehr (1967), Jost and Heindorff et al. (1976). At angles below 50 degrees there are serious discrepancies between several experiments. The optical-model calculation of McCarthy et al. (1977) predicts the angular behaviour for energies of 100 eV and above. The comparison with calculations using a real potential shows that the calculation of Walker (1971), which includes exchange, is to be preferred over that of Fink and Yates (1970), which neglects exchange effects.

A general approach for calculating cross sections for electron impact ionisation of atoms and ions with a single valence electron in either the ground or an excited state is presented. The differential energy exchange cross sections are based on the semiclassical treatments of Burgess (1963) and Vriens (1966). The model takes into account direct as well as electron exchange collisions, and also includes interference effects. The motion of an atomic electron in the nl state is represented by the semiclassical energy distribution function. The criterion of validity of the method is given, from this it follows that the theory is applicable to the calculation of the electron impact ionisation cross sections for H I, He II, Li III, C IV, N V, O VI, Ne VIII and Ne X. The present results are compared both with experimental data and with other theoretical calculations.

Differential and integral cross sections for elastic scattering and excitation of Ba and Mg atoms were calculated by solving the close-coupling equations including the ground and resonance target states. In the valence shell of an atom the configuration interaction was approximately taken into account by a simple procedure. The contribution of the highest partial waves to the cross sections was calculated either in the Bethe approximation (for excitation) or in its modification (for elastic scattering). The polarisation of the resonance lines radiation was also obtained. The energy dependence of the excitation cross section and polarisation in the region from threshold to 35 eV are presented for Ba. Comparison with experimental data shows that strict consideration of the configuration interaction is necessary. The effect of this interaction is most important in the region of the excitation cross section maximum.

Cross sections have been calculated for the scattering of slow electrons by water and methane molecules. A simple model is used: scattering is from a rigid molecule, exchange and polarisation effects are included in an approximate way, exchange by orthogonalising the scattered electron function to bound orbitals of the same symmetry and polarisation by using a potential correct at large distances but with an ad hoc short range cut-off parameter; the short range static interaction is, however, obtained from SCF target wavefunctions. Total, momentum-transfer and differential cross sections are given for CH₄. In this body-fixed approximation total cross sections are undefined for molecules with a permanent dipole moment such as H₂O and H₂S; results are given here for momentum-transfer and differential cross sections of H₂O. Possible negative-ion resonance states in H₂O and H₂S are discussed with reference to recent dissociative attachment and vibrational excitation experiments.

Electron energy-loss spectra have been obtained, with a resolution of 0.030 eV for the electronic states of benzene below 11.5 eV. At high scattering angles prominent vibrational structure has been observed in the first three triplet bands (³B_1u, ³E_1u, and ³B_2u). A progression of three levels starting at 6.10 eV has been observed for the first time, and assigned to the ³E_2g valence state. Rydberg states converging to the first and second ionisation potentials have been studied and in both cases quadrupole transitions have been detected for the first time. A series of four such levels converge to the first ionisation potential with a quantum defect of 0.266, and the Rydberg orbital is identified as nd(a_1g). Two quadrupole transitions converging to the second ionisation limit have a similar quantum defect of 0.226 and are believed to involve transitions from the 3e_2g orbital to the same Rydberg orbital.

Cross sections for the formation of mesonic hydrogen have been calculated in the Born approximation with the complete perturbation operator and its dipole part. The dependence of the cross section on the initial meson energy and final state quantum number has been studied. Primary populations of mesonic atom levels have been calculated taking the competition between the moderation and the atomic capture of mesons into account. The population distribution over n extends deep into the range of large n. The C_nl distributions have maxima at intermediate values of l=l₀ (dependent upon the meson mass) and fall off abruptly when l>l₀. The primary alignment of the angular momentum of the mesonic atom is close to the limiting value corresponding to the zero-order projection of the angular momentum.

The problem of two-photon resonant third-harmonic generation is analysed using a quantised field formalism. Photons are generated at the harmonic frequency as a result of transitions between sets of doublet states of the Autler-Townes type and hence, in the general case, necessarily have more than one spectral component.